1. Technical Field
The present disclosure relates generally to intervertebral implants and, more particularly, to an intervertebral implant having a composite wedge/dowel configuration suitable for interbody spinal fusion.
2. Background of Related Art
Intervertebral implants for fusing together adjacent vertebrae of a spinal column are well known in the surgical arts. Typically, a surgical procedure for implanting an intervertebral implant between adjacent vertebrae is performed to treat back pain in patients with ruptured or degenerated intervertebral discs, spondylolisthesis or other pathologies. A variety of different types of intervertebral implants have been developed for such a procedure including intervertebral wedge implants, spinal fusion cages and cylindrical threaded bone dowels.
A variety of different types of intervertebral implants have been developed to perform this function including spinal fusion cages, threaded bone dowels and stepped bone dowels. Exemplary implants are disclosed in U.S. Patent Applications filed on even date herewith, under Certificate of Express Mail Label Nos. EL260888076US and EL071686220US, and entitled xe2x80x9cRamp-Shaped Intervertebral Implantxe2x80x9d and xe2x80x9cKeyed Intervertebral Dowelxe2x80x9d, respectively, the entire disclosures of which are incorporated herein by reference.
One fusion cage described in U.S. Pat. No. 5,015,247 includes a cylindrical implant constructed from titanium having one closed end, one open end and a series of macro-sized openings formed through a side wall of the implant. The open end of the cylindrical implant is internally threaded and configured to receive a cap. A series of external threads are formed about the circumference of the cylindrical implant. Prior to use, a bone graft of cancellous bone taken from a patient""s iliac crest is placed in a press and forced into the hollow body of the cylindrical implant such that cancellous bone extends through the macro-sized openings. The cap is then screwed onto the internally threaded end of the implant. Subsequently, the cylindrical implant is screwed into a previously prepared receiving bed between two adjacent vertebrae.
Because of their simplicity, spinal fusion cages are widely accepted. However, spinal fusion cages suffer from several drawbacks. For example, the cylindrical loading surface area of spinal fusion cages is small, thus two spinal fusion cages are typically required during a surgical procedure. Secondly, spinal fusion cages are made primarily from metal, most notably titanium. This material does not remodel but remains in a patient forever or until it is removed. Since vertebral bodies eventually fuse with the cancellous bone or other bone growth material positioned within the fusion cage, if removal is required, it can be very difficult and dangerous to the patient. Thirdly, spinal fusion cages do not maintain lordosis, thus the natural curvature of the spine is altered. Finally, it is difficult to insert a spinal fusion cage and achieve equal purchase with the adjacent vertebrae. A spinal fusion cage will often tend to engage one vertebrae more securely than the other.
Wedge implants also suffer from several drawbacks. Although wedge implants have a greater load bearing surface area and are configured to maintain lordosis, wedge implants are more difficult to secure in place since they are not threaded into the vertebrae. Moreover, wedge implants have limited ability to prevent rotational forces between the two vertebrae that are intended to be fused.
Threaded bone dowels also suffer from some of the same drawbacks as spinal fusion cages. Threaded bone dowels have a small loading surface area and they do not maintain lordosis. Furthermore, threaded bone dowels are typically cut from bone with a hollow drill bit and subsequently are threaded. The hollow drill bit is positioned to cut transversely through the bone and the intramedullary canal during the cut. If the distance between the outer surface of the cut dowel and the intramedullary canal does not exceed a predetermined thickness, the dowel must be rejected. Since there is little bone to spare during such a transverse cut, a high percentage of bone dowels cut may be rejected due to anatomical variability between donors.
Accordingly, a need exists for an improved intervertebral implant which maintains simplicity for consistent surgical implantation, creates an improved biomechanical construct when implanted, maintains lordosis, conforms to vertebral endplates, spares the endplates in the load bearing region while perforating them in other areas to gain access to cells in cancellous bone; when produced from bone, can remodel into bone, can be easily manufactured and addresses other problems associated with current spinal fusion implants.
In accordance with the present disclosure, an intervertebral implant having a composite wedge/dowel configuration is provided. The intervertebral implant includes a central body portion and a pair of radially extending wings. The radially extending wings can be tapered from a first end of the implant to the second end of the implant along an axis parallel to the longitudinal axis of the cylindrical body portion for anterior or posterior insertion. Alternately, the radially extending wings can be tapered along an axis perpendicular to the longitudinal axis of the cylindrical body portion for lateral insertion or the wings can be tapered along any axis between axis parallel and perpendicular to the longitudinal axis of the implant. A throughbore or a plurality of throughbores extend from a top surface of the implant to a bottom surface of the implant providing a space for boney bridging to occur between the vertebrae which are intended to be fused. The throughbore(s) is dimensioned to receive growth factors including autograft, allograft, DBM, etc., to stimulate bone healing.
In a preferred embodiment, the implant is formed from a cortical ring allograft cut from the diaphysis or metaphysis of a long bone. The implant can be formed by milling the top and bottom surfaces of the cortical ring to form the central body portion and the tapered radially extending wings. The implant is milled such that the intramedullary canal of the cortical ring defines a throughbore in the central body portion of the implant. Thereafter, the sidewalls of the implant may be machined to form a substantially rectangular shape or be maintained in an essentially semi-circular configuration. Alternately, the implant may be formed of any biocompatible material having the requisite strength requirements via any known process, i.e., molding, casting, machining, etc.